An autoinjector is an automatic injection device designed to facilitate automated delivery of a dose of medicament to a patient through a hypodermic needle, the injection usually being administered by the patient themselves. An autoinjector works, for example, by delivering an injection automatically upon actuation by the patient pressing a button, moving a lever or part of a housing etc. This is in contrast to a conventional manual syringe where a plunger must be manually depressed into a barrel containing medicament in order to effect the injection. The terms “autoinjector” and “injection device” are used interchangeably in the following description.
Examples of autoinjectors are described in WO2003/099358 (Seedlings Life Science Ventures LLC) and WO01/93926 (Mayo Foundation for Medical Education and Research). These are both generally flat devices which are of small size to encourage users to carry the device with them for ready access. GB2396298 (PA Consulting Services Ltd) is an example of a more conventionally-shaped elongate autoinjector, but of relatively complex internal construction.
All three of the above prior art devices have a custom designed medicament chamber therein rather than being built around a standard pre-filled syringe presentation. The custom medicament chamber, although allowing for a compact overall size for the device, means that the device as whole must be subjected to more rigorous regulatory control as compared with a device containing a standard pre-filled syringe presentation which will have already obtained regulatory approval.
In general, an autoinjector includes a needle which is located within the housing of the device. Upon activation of a force-generating source, a portion of the needle extends out of the housing and penetrates the outer layer of skin to deliver medicament. In some known autoinjectors, after activation, a needle cover or needle shield moves forward to conceal the needle after use. In GB2396298, the needle automatically retracts back into the housing by means of a biasing spring.
An improved autoinjector is described in our international patent application, published under number WO 2005/070481. This device requires that the needle is moved axially so that it can appear beyond the end of the nozzle for the duration of the injection, after which the needle retracts automatically, so that it is never in sight of the user. The device also requires that the plunger is moved axially so that medicament is ejected. The overall complexity of the autoinjector is significantly reduced by both of these requirements being effected by one component, namely an inner housing, and the device has the significant advantage that it can be built around a conventional or standard syringe presentation.
The injection device of WO 2005/070481 is designed to be used in conjunction with a standard drug presentation e.g. a pre-filled syringe comprising a needle, a barrel pre-filled with medicament and a plunger. The plunger may include a separately-provided plunger rod. As mentioned above, there is a significant commercial advantage in being able to use a standard pre-filled syringe, which will have been subjected to numerous clinical trials, drug stability studies and regulatory approval. Any modification to the standard syringe may require further trials and approval, adding delay and expense.
An improved autoinjector device is described in WO 2009/127861 (The Medical House Limited) wherein the needle does not automatically retract back into the device immediately after a dose of medicament has been delivered. Instead, in the device of WO 2009/127861, the user selectively actuates the retraction of the needle when desired. This may be by removing the device from the injection site, or by actuating some other trigger means. It is known that, in some cases, automatic retraction is undesirable as excessively quick retraction of the needle after delivery may cause the medicament to escape from the puncture hole created by the needle in the tissue. This is especially true if the liquid has not dispersed into the tissue and is in a “pressurised bolus” at the point of delivery which is a known problem when injecting adrenaline, for example. The device of WO 2009/127861 therefore mitigates these problems by allowing the retraction of the needle to be manually controlled or at least delayed.
It is an object of the present invention to provide an alternative and/or improved solution to the above-mentioned known problems associated with automatic retraction autoinjector devices.